The United States is the largest mining country in the world. In 2001, the mining industry produced $57.3 billion in raw materials, of which $19.0 billion was derived from coal. The mineral processing industries increased the value of the minerals to $374 billion, while coal and uranium were used to produce $147 billion of electricity. Thus, the minerals and coal industries combined to contribute $521 billion to the nation's wealth (approximately 5.2% of the Gross National Product).
A major problem faced by the coal industry is environmental concerns created during coal production. According to a National Research Council report, the U.S. coal industry discards 70-90 million tons of fine coal annually to approximately 713 active impoundments. There is accordingly a need in the art for improved processes for processing fine coal to improve recovery of potentially useful products from such waste materials and reduce the need for discarding into waste piles and fine coal impoundments.
The industry has placed emphasis on improvements in physical separations of valuable substances from undesired substances. As a general rule, separation efficiencies decrease as the size of the particle being separated decreases. Methods evaluated to date may be considered to fall into three categories: (1) size-size separations (screening, classification); (2) solid-solid separations (flotation, selective flocculation, magnetic/electrostatic separation, gravity separation); and (3) solid-liquid separations (thickening, centrifugation, filtration, drying).
The most common classifier used for fine particle classification in mineral processing applications is the hydrocyclone, which is commonly used to separate particles as fine as 75 μm. Although separations as fine as 45 μm can be achieved, to do so the geometry of the hydrocyclone becomes much smaller and the capacity is reduced. Capacity limitations can be remedied by increasing the number of cyclones used, but the small apertures necessary are prone to plugging. As such, hydrocyclones are not suitable for efficient classification of ultrafine particles.
It is known to use hydraulic classifiers to separate particulates from a slurry by gravity sedimentation. Such hydraulic classifiers are designed primarily to de-slime materials, and place an emphasis on achieving clean coarse fractions. In general, conventional hydraulic classification is considered to be relatively ineffective on particles having a size <35 μm. Therefore, use of hydraulic classifiers in, for example, removal of carbon from fly ash being separated (beneficiated) for use in, e.g., cement or concrete as a mineral admixture has received limited attention.
Conventionally designed hydraulic classifiers provide a trough-shaped body defining one or more cells, and may optionally include substantially upright dividers of varying heights separating each cell. Each cell includes an outlet or underflow near the bottom thereof for removing particles as settling occurs. In use, a feed flow is established across the classifier. The largest particles will separate and can be removed from the first underflow, and so on. The finest particles will pass through the system, and may be discarded or collected, such as in a launder placed at an end of the classifier.
Such conventional hydraulic classifiers suffer from the disadvantage of inability to efficiently sort particle sizes of <7 μm. Further, blanket settling, a phenomenon wherein in a mix with differing particle sizes all of the particles settle concomitantly due to larger particles entraining smaller particles and hindering their movement, is a known disadvantage of such classifiers. Still further, most ores or other materials subject to hydraulic classification do not operate in the <10 μm range. Most ores are “deslimed” at anywhere from 200 to 325 mesh (74-45 μm). Coal fines, for example, are typically in the 100-200 mesh (150-74 μm) size range. The slimes, which may include potentially valuable fine particles, are considered a waste product in most mineral processing circuits.
As noted above, hydraulic classifiers having substantially vertical dividers (weirs) of differing heights are known also, and have been evaluated to attempt to improve selectivity for finer particles. However, such classifiers disadvantageously create thick sediment compression zones at or near the dividers, which hinders particle movement and size sorting and therefore efficiency, and results in relatively low throughput. Discharging settled solids from a single withdrawal point results in a high proportion of water also being withdrawn, thus creating disturbances in the bed of settled solids and short-circuiting of fine particles in the feed into the underflow. This limitation has been addressed by some design improvements that incorporate an elongated cone-shaped bed of settled solids that tapers to a discharge point at sufficient depth from the settling zone such that withdrawal of settled solids does not disturb particle sorting in the settling zone. However, with such a withdrawal geometry, fine particles entrained in the settled solids will be withdrawn with the coarse settled solids. Accordingly, conventional hydraulic classifiers are simply unsuited for sorting particle sizes of <7 μm.